1. Field of the Invention
The invention relates to a method for detecting a dot of functional material on a medium at a predetermined location, the dot having a predetermined color, wherein the method comprises the steps of determining an environment of the predetermined location, comprising a plurality of pixels surrounding the predetermined location and including the predetermined location itself, scanning the environment resulting in scanning values for each pixel of the environment, and for each pixel of the environment, establishing a value for a lightness component of a color of the pixel derived from the scanning values.
2. Description of Background Art
Image reproduction apparatuses are known, which are able to print jobs arriving at the image reproduction apparatus via a network or an analogue document via a scanner being part of the image reproduction apparatus. Such a job may contain an image or a text or both an image and a text in black-and-white format or in color format. The job entry in the image reproduction apparatus may be controlled by a controller, for example a computer, a control unit or a processor inside the image reproduction apparatus. Also the controller may convert image and text data into commands for the print unit to let the print elements eject functional material at the right location and the right time on the receiving material. The memory of the image reproduction apparatus comprises a work memory part for loading and modifying images and a save memory part for saving images. Instead of the term printing element, the term print element or nozzle may be used hereinafter.
However, nozzles may fail when they become clogged or are misdirecting.
Detectors are known, which can detect such a failing print element during printing or which can predict a high probability that a print element will fail in short time. The visibility of a failing print element on the receiving material depends on the print strategy. In a multi-pass approach, a failing print element appears typically less visible than in a single pass approach. In a multi-pass approach each pixel line is addressed by multiple print elements and a failing print element may be compensated for by filling in with another print element, for example in a later pass. However, such a print element failing correction for a multi-pass approach will not be possible in a single pass approach, where each pixel is addressed by only one print element.
In a single pass approach the failing print element immediately produces a light stripe in the print image on the receiving material and there is no chance to fill in this location later by means of another print element.
If one or more printing elements are detected as failing, a corrective action may be taken. The printing element failing detection may be applied on actual information of images which are to be printed, but may also be applied on spit patterns, which are additional to the actual information. A spit pattern is an arrangement of dots of functional material on the medium. The dots of the spit pattern are printed by different printing elements in order to check the state of the printing element, for example “ejecting” or “failing” may be a result of the detection.
Normally, the detection of a dot of a spit pattern is achieved by using a detector like a scanner, which can establish the lightness of a location on the medium, which is predetermined to receive the dot. Since the detector is less accurate, the spitting element is less accurate and the scanned image is smearing a dot over a larger area than one location, an environment of the predetermined location may be investigated and lightness components for each pixel in the environment may be established. A detector may use an RGB detection, an L*a*b* detection or an XYZ detection method. If a detector uses RGB detection, a channel out of the R, G and B channel may be selected for establishing the lightness. Which channel is used depends on the color of the functional material being measured. Using only a single channel for detection will deliver a reliable detection method. Normally a channel that provides the most contrast is selected.
A lightness based detector does not work well for all types of functional material like inks in combination with a medium. For example, magenta ink which by itself is already difficult to detect due to a poor contrast which it has in detector channels is very difficult to detect on paper with rough fibers and therefore with a strong fiber visibility on a scan file. For optimal operations, a detector must be balanced so that it only detects dots if they are present and must also give a negative result if no dot is present. As it turns out from experiments, for kinds of paper that have fibers in it that are visible in a scanned image of the paper, it is impossible to tune the detector so that it provides balanced results. This is due to the visibility of the fibers being present in such kind of paper. Either the detector is set to a value that magenta ink dots are found—but then there is large chance that the detector misfires on fibers in the paper—or the detector is set to a working point so that it does not trigger on the fibers in the paper—but in this case, the detector starts to miss a significant amount of magenta dots.
In other words, a disadvantageous result is that sometimes a dot is established that is not present or a dot is present that is not established. This leads also to a detection of a printing element that is not failing or to a non-detection of an actually failing printing element.